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Hydrological management of a heavily dammed river basin: the Miño-Sil
Juan A. Añel Mohcine Bakhat Xavier Labandeira
Hydrological management of a heavily dammed river basin: the Miño-Sil
Juan A. Añel1,2,3, Mohcine Bakhat3, Xavier Labandeira3,4,5
1 Smith School of Enterprise and the Environment, University of Oxford, OUCE, South Parks Road, OX13QY Oxford (UK) 2 EphysLab, Facultade de Ciencias, Universidade de Vigo, Campus As Lagoas, 32004 Ourense (Spain) 3 Economics for Energy, Doutor Cadaval 2, 3E, 36202 Vigo (Spain) 4 Rede, Universidade de Vigo, Facultade de CC.EE., Campus As Lagoas, 36310 Vigo (Spain) 5 FSR-Climate, European University Institute, Via delle Fontanelle 19, 50014 Firenze (Italy)
We herein research the potential environmental impacts of the management of dams in the Miño-Sil river basin on the natural flow of their rivers. The Miño-Sil is a transnational river basin in the north-western Iberian Peninsula, and is managed by Spanish authorities. The basin is heavily managed with more than 100 dams, which in the main are used exclusively for hydropower generation. For the period of this study (1978-2012), we analyze the repercussions of the liberalization of the Spanish energy market in 1998. Our results show that the dams in the Miño-Sil river basin years had no influence on the natural river flows over the period of interest. Moreover, despite being used so heavily for hydropower, the liberalization of the Spanish energy market did not increase the degree of intervention in river flows. Indeed for three reservoirs in particular the correlation between inflow and outflow improved. It is also clear that for the reservoirs considered, the mean water storage and monthly inflows were lower during 1998-2012 than during 1978-1997.
Keywords: water management, reservoirs (surface) dams, hydroclimatology, systems operation and management
___________________ Corresponding author: Juan A. Añel. Email: [email protected] Phone: +44 (0)1865 614940 The authors would like to thank the Confederación Hidrográfica Miño-Sil (the Miño-Sil river Basin District Authority) for providing some of the data used for this study. We would also like to thank Carlos G. Ruiz (MSRBDA) and David A. Pérez (Gas Natural Fenosa) for their useful comments and help. The usual disclaimer applies. The funders had no role in the design of the experiments, analysis, or interpretation of results. This study was partly funded through a contract with the Gas Natural-Fenosa Chair at the Universidade de Vigo. The funder had no role in the research, the design of the experiments, or the analysis and interpretation of the results.
The presence of a dam can significantly alter the hydrological characteristics of a catchment. They are
built for a range of purposes, including flood control, hydropower generation and water supply. Their
environmental impact depends on their use, on geophysical factors (meteorology, climatology), and on
their management strategies. There is a significant scientific literature on the effects of dams on floods
[e.g., López-Moreno et al., 2002], droughts [e.g., López-Moreno et al., 2009], and the environment [e.g.,
Garcia et al., 2011]. The common cause of all these phenomena is the alteration of natural flows as a
consequence of the existence and operation of dams [e.g., Botter et al., 2010]. Several authors have
correspondingly proposed the use of operating rules to minimize the negative effects of dams [Renöfalt et
al., 2010; Yin et al., 2011], although these may compromise other objectives pursued by dam builders
[Barlett et al. 2012].
The 'Miño-Sil basin' is the common name for the area administered by the Miño-Sil River Basin District
Authority (MSRBDA). The area comprises the basins of the Miño, Sil, Cabe and Limia rivers, and is
divided into six different zones: Miño-Alto, Miño-Bajo, Cabe, Limia, Sil-Superior and Sil-Inferior (see Figure
1). The Miño-Sil is the fourth largest basin in the Iberian Peninsula in terms of annual mean flow (10570
hm3), and is thus important for hydropower generation [Lorenzo-Lacruz et al., 2012]. The Miño river flows
into the Atlantic Ocean: 95% of its basin is in Spain, but its last 76 km form the border with Portugal. The
Limia crosses the border with Portugal, while the Sil and Cabe lie entirely in Spanish territory. The
MSRBDA manages the Spanish sections of the Miño and Limia rivers, together with the transitional and
coastal waters shared with Portugal, under a bilateral agreement [Ministerio de Asuntos Exteriores y de
The Miño-Sil basin is economically important for the region, in the tourism, fishing, agricultural and power
generation sectors. One main characteristic of the Miño-Sil basin is that most of the reservoirs are used
solely for hydropower generation, with just two also used for water supply or irrigation (see Table 1). There
are 106 hydroelectric stations in the basin, of which 36 are cataloged by the MSRBDA as 'big reservoirs'
on the basis of their total surface areas. In 2012 the installed generating capacity reached 2773 MW.
Water from the basin is also used by two thermal power plants with an installed capacity of 1629 MW,
bringing the basin’s gross annual production of electricity to 14143 GWh, constituting 8.58% of the Iberian
Peninsula’s total electrical installed power [Confederación Hidrográfica del Miño-Sil, 2012] (information
available online in www.chminosil.es). Several further projects have recently been finished or are under
development to increase hydropower generation, including pumped storage reservoirs for use with
existing local wind power plants.
For such a complex and heavily modified basin [Martínez-Gil and Soto-Castiñeira, 2006], one which
already contributes a major share of Spanish electricity generation capacity and which is due to be
increased through the addition of further hydropower projects, it is very important to gain a quantitative
insight into its management from an environmental point of view in order to understand the effects of
hydropower as mentioned above. Accordingly, in 2009 the regional government of Galicia, where most of
the dams in the Miño-Sil basin are located, introduced a tax on hydropower production. Even though the
tax was probably aimed at capturing the hypothetical rents associated with hydroelectric generation [see
Gago et al., 2013], it may also have reflected environmental concerns.
The operation of a hydropower system depends primarily on the availability of water resources (runoff),
but it also depends on other technical guidelines (for example those provided by the operator of the grid)
and the economic factors related to the alternatives to hydropower generation. In the face of scarce or
non-existent information on these secondary aspects, the potential changes in electricity generation from
existing hydropower plants can be considered to be a consequence of the impact of climate change for a
country or region.
Instead of a single managing body, the Miño-Sil basin relies on a mix of decisions made by public
authorities, including the MSRBD and Red Electrica de España (the operator of the Spanish electricity
system), and several independent private companies that use the hydropower stations (see Table 1). It is
likely that these decisions are motivated by a range of motivations, including economic factors, in addition
to the region’s climatology and hydrometeorology.
It has been shown that climatology is important, because of the link connecting the annual hydrological
cycle with phenomena such as the North Atlantic Oscillation (NAO) [García et al. 2005; Gimeno et al.
2001; Trigo 2011]. In addition to the usual effects of climatic variability on the region, climate change has
affected fresh water resources via changes in the hydrological cycle. The last report by the
Intergovernmental Panel on Climate Change [IPCC, 2013] supported the proposition of a positive trend in
precipitation for mid-latitude areas of land in the northern hemisphere. Bates et al.  argued that the
changes will be widespread and that: a) the quantity, variability, timing, form, and intensity of precipitation
and annual average runoff will change, b) the frequency and intensity of extreme events such as floods
and droughts will rise, c) water temperatures and the rate of evaporation will increase, and d) water quality
in rivers and lakes will deteriorate. Despite international agreement on the threat of climate change to
water resources, the nature and magnitude of these impacts are deemed to be country/region-specific.
Some regions are expected to receive too much or too little water, with a higher variability in precipitation
and river discharge projected to cause great damage due to an increase in floods and droughts. These
problems will be further exacerbated in the second half of the 21st century [Bates et al., 2008].
Mukheibir  makes a distinction between long-term impacts related to trends, and short-term
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